Battery with high electrode interfacial surface area

ABSTRACT

An electrochemical battery cell in accordance with the invention has a high electrode interfacial surface area to improve high rate discharge capacity, and the shapes of the electrodes facilitate the manufacture of cells of high quality and reliability at high speeds suitable for large scale production. The interfacial surfaces of the solid body electrodes have radially extending lobes that increase the interfacial surface area. The lobes do not have sharp corners, and the concave areas formed between the lobes are wide open, to facilitate assembly of the separator and insertion of the other electrode into the concave areas without leaving voids between the separator and either electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/251,002, filed Sep. 20, 2002, entitled Battery with High ElectrodeInterfacial Surface Area, currently pending, which is herebyincorporated by reference.

BACKGROUND

This invention relates to electrochemical cell batteries, particularlyto cells with increased interfacial surface area between the positiveand negative electrodes.

Batteries containing electrochemical cells are used as power sources forelectrical devices. An ideal battery would be one that is inexpensive,with unlimited capacity regardless of power level, temperature oroperating conditions. It would also have an unlimited storage life, besafe under all conditions, and be impossible for the user to misuse orabuse. While such an ideal battery is not possible, batterymanufacturers continue to design batteries that will come closer to thatideal. In a practical battery, there are tradeoffs and compromises thatmust be made among the ideal battery characteristics related to batteryperformance. Thus, the requirements of the electrical devices that willbe powered by the battery are important factors in battery and celldesign. For example, many devices have battery compartments that limitthe size and shape of the battery or batteries, and the dischargecharacteristics of the battery/batteries must be sufficient to operatethe device under expected conditions of use.

Manufacturers are continually trying to increase the capabilities andthe number of features of electrical devices. This results inever-increasing demands for batteries that will provide higher powerwithout unacceptable sacrifices in the other desirable batteryperformance characteristics, such as long discharge life (highcapacity), long storage life, resistance to leakage, and ease ofmanufacture. This trend in increasing power requirements is evident inportable devices with consumer-replaceable batteries.

Achieving high battery capacity and long discharge life is especiallychallenging at high discharge rates required for high power becausebatteries are able to deliver only a fraction of their theoreticalcapacity, and that fraction (the discharge efficiency) decreases as thedischarge rate increases. There are many factors that contribute to thedischarge efficiency of batteries and the cells they contain. One factoris the interfacial surface area between the electrodes. Increasing theinterfacial surface area generally has positive effects on currentdensity, internal resistance, concentration polarization, and othercharacteristics that can effect discharge efficiency. However,increasing the interfacial surface area often comes at the expense ofreduced active materials and theoretical discharge capacity. Indesigning a cell with increased interfacial surface area it is desirableto minimize necessary reductions in active materials, increases in inertcomponents, and increases in expensive materials that do not themselvesimprove performance, as well as any other changes that reduce thetheoretical capacity or otherwise offset improvements.

Some consumer batteries use active materials and/or electrolytes thatare especially well suited for high power applications. Examples includeprimary lithium batteries and rechargeable (secondary) nickel/cadmiumbatteries. These batteries often use materials that are relativelyexpensive, have special handling requirements, or raise environmentalconcerns in the disposal of spent batteries. Because high interfacialsurface area is generally preferred for high rate/high powerapplications, these batteries often have spiral wound electrode designs.However, these designs usually have more internal volume consumed byseparators and current collectors and are generally more difficult andexpensive to manufacture than bobbin designs.

The use of alkaline zinc/manganese dioxide batteries can solve theseproblems if the device does not exceed their power requirements. Thereis a need to improve the high power capability of alkaline batteries tomake them suitable as power sources for higher power devices.

In a cylindrical alkaline Zn/MnO₂ cell with a bobbin-type construction,high rate discharge performance can be improved by increasing theelectrode interfacial surface area. Typical commercial cells of thistype have a positive electrode disposed next to the can. This positiveelectrode (cathode) has essentially a hollow cylindrical shape with asmooth, round internal surface, within which the separator and negativeelectrode (anode) are disposed. The electrode interfacial surface areacan be increased by changing the internal surface of the positiveelectrode so that it is no longer smooth. One convenient way to do this,which is compatible with typical cell manufacturing processes in use, isto corrugate the positive electrode surface, with the corrugationsrunning vertically (i.e., parallel to the can side walls when thepositive electrode is assembled into the can). In general, the higherthe surface area, the better the high rate discharge capacity.Additional improvement in high rate discharge capacity may also berealized if the cathode thickness is generally reduced, since this willtend to reduce polarization of the positive electrode.

There have been previous attempts to improve the high power capabilityof alkaline batteries by increasing electrode interfacial area. Examplescan be found in U.S. Pat. No. 5,869,205, No. 6,074,781 and No.6,342,317. However, each of these references suffers from one or more ofthe following disadvantages.

Manufacture of cells is difficult when a current collector prong mustextend into each of a plurality of like-polarity electrodes. This meansthat each current collector prong must be aligned with one of theplurality of electrodes, requiring orientation of both the cell and thecurrent collector. In addition, when multiple current collector prongsare required, the volume of active materials must be reduced to allowfor an increase in the total volume of the collector, compared to celldesigns in which a single current collector prong will suffice.

The use of typical separator materials (e.g., polymeric film and wovenor nonwoven paper or fabric) in strip or sheet form may be impracticaldue to difficulty in making the separator conform to the surface of thecavity in the cathode. Even application of a spray-on separator to theinterfacial surface of one of the electrodes can be difficult. Sharpcorners and non-vertical interfacial surfaces can also make it difficultto completely fill the cavity with anode at the high speeds desirable inmanufacturing.

When discharge efficiency is maximized by making the maximum distance ofactive material in a first electrode from an interfacial surface of asecond electrode, the resulting first electrode shape can create variousproblems during cell manufacture: (1) difficulty in inserting theseparator so that the entire interfacial surface of the first electrodeis covered by separator without leaving voids between the electrode andthe separator, (2) difficulty in keeping the separator against the firstelectrode surface so gaps do not develop before, during or afterinsertion of the second electrode, and (3) preventing the formation ofair pockets between the second electrode and separator during high speedcell assembly. Such electrode shapes also tend to include relativelyfragile lobes or projections extending from the electrode surfaces,making breakage more likely during electrode forming and handling, aswell as during and after assembly of the electrodes and separator intothe cell container.

The smaller diameter cells (e.g., AA/R6 and AAA/R03 sizes) areparticularly susceptible to the above problems due to more the morelimited spaces available and the need to make the electrode dimensionssmaller.

In view of the above principles, an object of the present invention isto provide an electrochemical battery cell that is inexpensive and easyto manufacture, has high capacity, performs well under expectedtemperature and operating conditions, has long storage life, is safe,and is not prone to failure as a result of misuse or abuse by the user.

Another object of the present invention is to provide a battery cellthat has improved high rate/high power discharge performance withminimal adverse effects on theoretical capacity, discharge performanceat moderate and low rates, and other desirable battery cellcharacteristics.

It is also an object of the present invention to provide an economicalbattery cell with electrodes having a high interfacial surface area.

In view of the above problems with cell designs having high electrodeinterfacial surface area, it is a further object of the presentinvention to provide an economical, reliable alkaline zinc/manganesedioxide battery cell with a bobbin-type electrode configuration, capableof high speed mass production, that has a high electrode interfacialsurface area.

SUMMARY

The above objects are met and the above disadvantages of the prior artare overcome by an electrochemical battery cell of the presentinvention. The present invention is directed to an electrochemical cellbattery. The cell comprises a housing with an upstanding side wall, afirst electrode comprising a first active material, a second electrodecomprising a second active material and disposed within the firstelectrode, a separator disposed between the first and second electrodes,and an electrolyte. At least one of the first and second electrodescomprises a solid body, a surface of which defines a surface of a cavityin which the other of the first and second electrodes is disposed. Thesurface of the cavity comprises a plurality of radially extending lobesthat form a plurality of concave and convex areas in the surface of thesolid electrode body, each convex area has no radius less than 0.030inch (0.76 mm), and each concave area has no radius less than 0.030 inch(0.76 mm).

An embodiment of the invention may have a third electrode which alsocomprises a solid body. In such an embodiment the third electrode may beof the same polarity as the first electrode and be disposed within thesecond electrode such that the first and third electrodes define thecavity within which the second electrode is disposed. The firstelectrode comprises an external surface of the cavity, and the thirdelectrode comprises an internal surface of the cavity. In another suchembodiment the third electrode may be of the same polarity as the secondelectrode and be disposed outside the first electrode such that thesecond and third electrodes define the cavity within which the firstelectrode is disposed. In this embodiment the third electrode comprisesan external surface of the cavity, and the second electrode comprises aninternal surface of the cavity.

In another embodiment of the invention the first electrode is a solidbody, the external surface of which has a shape conforming to a shape ofthe upstanding wall of the housing. The internal surface of the firstelectrode has radially inward extending lobes that form concave areas.The second electrode is disposed within the cavity in the firstelectrode and has an external shape defined by a shape of the cavity inthe first electrode and the separator.

In another embodiment the second electrode is a solid body, and theexternal surface of the second electrode and the upstanding wall of thehousing define the cavity within which the first electrode is disposed.The second electrode comprises a plurality of radially outward extendinglobes forming a plurality of concave areas in the external surface ofthe second electrode. The external and internal shapes of the firstelectrode are defined by a shape of the upstanding wall of the housingand by an external shape of the external surface of the second electrodeand the separator, respectively.

In yet another embodiment of the invention the first electrode comprisesa solid body with a minimum radial thickness d₂. Each lobe of the firstelectrode has a width d₁, perpendicular to the radial center line of thelobe at a radial distance half way between the base and the end of thelobe. The ratio d₁:d₂ is greater than 2.5:1 but not greater than 8.1:1.

Another embodiment of the invention is a primary electrochemical cellbattery comprising a housing with an upstanding side wall, a firstelectrode comprising a first active material comprising manganesedioxide, a second electrode comprising a second active materialcomprising zinc, a separator disposed between the first and secondelectrodes, and an electrolyte comprising an aqueous solution ofpotassium hydroxide. At least the first electrode is a solid body, theinternal surface of which defines a surface of a cavity in which thesecond electrode is disposed. The surface of the cavity comprises aplurality of radially extending lobes that form a plurality of concaveand convex areas. Each solid body electrode lobe has a convex surfacewith no radius less than 0.030 inch (0.76 mm). No concave area widthincreases as the radial distance from its base to its open endincreases. For each first electrode lobe a ratio of the lobe width (asmeasured between two points on the surface of the first electrode, eachof those points being a radial distance from the longitudinal axis ofthe cell equal to the average of the radial distance from thelongitudinal axis to the outermost internal surface of the firstelectrode and the radial distance from the longitudinal axis to theinnermost internal surface of the first electrode) to the minimum radialthickness of the first electrode is at least 2.5:1 but not greater than8.1:1.

Among the advantages of the present invention is facilitation of highspeed manufacturing of electrochemical battery cells with high electrodeinterfacial surface area. This is accomplished when radii in the concaveinterfacial surfaces of the solid electrode bodies are not less than0.030 inch (0.76 mm). This avoids corners that are too tight to properlyinsert or apply separator or to properly fill with material of the otherelectrode at high manufacturing speeds. If separator and material of theother electrode are not properly inserted into these concave areas,there may be gaps that can increase the cell internal resistance,increase the current density and concentration polarization elsewhere,and reduce the discharge efficiency. There may also be damage to ornonuniform or insufficient coating of separator at the interface betweenthe electrodes, which can result in nonuniform discharge or internalshorts during cell manufacture or use. Avoiding tight corners can alsohelp to keep gaps from developing between the separator and electrodesduring and after assembly. Separator materials may tend to spring backinto their previous shape, which may not conform precisely with theshapes of the electrodes at their interfacial surfaces, causing gaps tooccur even if there were no gaps initially.

The advantages of the present invention are also realized when, in eachsolid body outer electrode lobe, the perpendicular distance from theradial center line to the surface of the lobe increases as the radialdistance to the ends of the adjacent lobes increases. This alsofacilitates assembly of the electrodes and separator and the avoidanceof gaps and cell defects, in a manner similar to that described above.

These and other features, advantages and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims and appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a cross-sectional view of a conventional battery cell takenalong the longitudinal axis of the cell.

FIG. 2 is a cross-sectional view of the battery cell in FIG. 1, taken atII-II.

FIG. 3 is a cross-sectional view, of a first battery cell with highelectrode interfacial surface area.

FIG. 4 is a cross-sectional view of a second battery cell with highelectrode interfacial surface area.

FIG. 5 is a cross-sectional view of a third battery cell with highelectrode interfacial surface area.

FIG. 6 is a cross-sectional view of a fourth battery cell with highelectrode interfacial surface area.

FIG. 7 is a cross-sectional view of a fifth battery cell with highelectrode interfacial surface area.

FIG. 8 is a cross-sectional view of a sixth battery cell with highelectrode interfacial surface area.

FIG. 9 is a cross-sectional view of a seventh battery cell with highelectrode interfacial surface area.

DESCRIPTION

Unless otherwise defined herein, the meanings of words used in thisspecification will be their normal meanings as understood by thoseskilled in the art of electrochemical battery cells. The followingdefinitions and relationships are used unless otherwise specifiedherein:

-   “solid electrode body” means an electrode that, upon assembly into    the cell, is capable of maintaining the shape of its surface that    interfaces with another electrode in the cell through the separator    without the support of a current collector, separator or other    electrode; a solid electrode body does not include gelled    electrodes;-   “internal,” “external,” “inner” and “outer” are relative to the    longitudinal axis of the cell; if the cell is not symmetrical, the    longitudinal axis passes through the centers of area of cross    sections normal to the lengthwise dimension of the cell at the    lengthwise extreme of the electrodes;-   “lobe” means a projection from the surface of an electrode, not    including normal variability in the electrode surface due to the    nature of the component materials, the porosity of the electrode and    the like;-   “radial center line” means a line radiating from and normal to the    longitudinal axis of the cell;-   “interfacial surface” means the surface of an electrode that is    adjacent the opposite polarity electrode through the separator, and    “interfacial surface area” means the total area of the interfacial    surface, as measured at the solid electrode (if both electrodes are    solid, the interfacial surface area between the two solid electrodes    is the larger of the two; if there is a third electrode, the    electrode interfacial area of the cell is the sum of the interfacial    surface areas between electrodes of opposite polarity in the cell);-   “electrode volume” is the volume contained within the boundary    surfaces of the electrode, including pores in the electrode;-   “lobe base” is that part of a lobe where the lobe meets adjacent    lobes;-   “lobe tip” is the end of the lobe away from its base;-   “open end” is that part of a concave area in an electrode surface    that is between the tips of the lobes forming that concave area;-   the first electrode is disposed radially outside the second    electrode; the third electrode, when present, may be either the    innermost or the outermost electrode; and-   dimensions shown in the drawings are:-   r₁—the radial distance from the center of the cell to: the outermost    internal surface of the first electrode when the first electrode is    a solid body, whether or not the second electrode is also a solid    body, and the outermost external surface of the second electrode    when the second electrode is a solid body and the first electrode is    not;-   r₂—the radial distance from the center of the cell to: the innermost    internal surface of the first electrode lobes when the first    electrode is a solid body, whether or not the second electrode is    also a solid body, and the innermost external surface of the second    electrode when the second electrode is a solid body and the first    electrode is not;-   d₁—the width of a first electrode lobe, as measured between two    points on the surface of the lobe that are each a radial distance    (r₁+r₂)/2 from the center of the cell;-   d₂—the minimum radial thickness of the first electrode;-   R_(1a)—the smallest radius of the concave portion of the inner    surface of the first electrode;-   R_(1b)—the smallest radius of the convex portion of the inner    surface of the first electrode;-   R_(2a)—the smallest radius of the concave portion of the outer    surface of the second electrode;-   R_(2b)—the smallest radius of the convex portion of the outer    surface of the second electrode;-   w₁—the perpendicular distance from the center line of a first    electrode lobe to a surface of the lobe; and-   w₂—the perpendicular distance from the center line of a second    electrode lobe to a surface of the lobe.

Referring to FIG. 1, a conventional electrochemical battery cell 10 isshown. Cell includes a housing, comprising a can with a side wall 12, aclosed bottom end 14, and an open top end 16. A first terminal cover 18is welded or otherwise attached to can bottom 14. Alternatively, canbottom 14 may be formed to include the shape of first terminal cover 18in order to function as the first terminal and eliminate the need for aseparate cover. Assembled to the open top end 16 of the can is a coverand seal assembly with a second terminal cover 30. A plastic film label20 or other jacket may be formed about the exterior surface of the canside wall 12. Label 20 may extend over the peripheral edges of first andsecond terminal covers 18 and 30. A first (outer) electrode 22 is formedabout the interior surface of the can. First electrode 22 is in directcontact with a portion of the can, and the can functions as firstcurrent collector, providing electrical contact between first electrode22 and first terminal cover 18. A second (inner) electrode 26 isdisposed within a cavity in first electrode 22, with a separator 24between first and second electrodes 22 and 26. A second currentcollector extends from second terminal cover 30 into second electrode 26to provide electrical contact between second electrode 26 and cover 30.An annular seal 32 is disposed in the open end 16 of the can to containthe electrode materials and electrolyte in the can. An inner cover 34provides compressive support for seal 32 to achieve the desired level ofresistance to leakage of materials from cell 10. Seal 32 alsoelectrically insulates second terminal cover 30 from the side wall 12 ofthe can.

FIG. 2 is cross-sectional view of the cell 10 in FIG. 1 taken at II-II.In a typical alkaline zinc/manganese dioxide cell, first electrode 22comprises a solid body. First electrode 22 has a cylindrical shape withan internal surface that is generally smooth. The external surface offirst electrode 22 conforms generally to the shape of the internalsurface of can side wall 12. The external surface of first electrode 22may be in direct contact with can side wall 12, or it may be spacedapart from side wall 12, e.g., with a sheet of electrically insulatingfilm. First electrode 22 also has an internal surface, which defines acavity. Second electrode 26 is disposed within the cylindrical cavitydefined by the internal surface of first electrode 22. Separator 24 isdisposed between first electrode 22 and second electrode 26, as shown inFIG. 1. In a typical alkaline zinc/manganese dioxide cell, secondelectrode 26 is not a solid body, but comprises a flowable material,such as a liquid or a gel, and must be inserted into the cell afterfirst electrode 22 and separator 24.

FIG. 3 is a cross-sectional view, similar to that in FIG. 2, but of anelectrochemical battery cell 110 with an increased electrode interfacialsurface area. Instead of having a smooth cylindrical inner surface, likethat of first electrode 22 of conventional cell 10, first electrode 122of cell 110 comprises a plurality of lobes 123 that project radiallyinward. In contrast to cell 10, in which the internal surface of firstelectrode 22 has a round cross section, lobes 123 of cell 110 serve toincrease the perimeter length of the internal surface of first electrode122. This increases the area of the internal surface of first electrode122 where it interfaces with second electrode 126 through separator 124.Adjacent lobes 123 create concave areas in the internal surface of firstelectrode 122. Second electrode 126 has an external shape that isgenerally defined by the shape of the internal surface of firstelectrode 122 in combination with separator 124. In embodiments wheresecond electrode 126 is a solid body, second electrode 126 has aplurality of lobes 127 that extend radially outward.

When first electrode 122 is a solid body, each of lobes 123 may have thesame dimensions, as shown in FIG. 3, or lobes 123 may have differentshapes and/or sizes. Each of the concave areas between adjacent lobes123 of first electrode 122 comprises at least one radius R_(1a). FIG. 3shows a single radius R_(1a) of the same size in each concave areabetween lobes 123, but in other embodiments the concave areas may eachcomprise more than one radius R_(1a). Regardless of the number of radiiR_(1a) in each first electrode concave area, no radius R_(1a) is smallerthan 0.030 inch (0.76 mm). This facilitates the proper insertion of bothseparator 124 and second electrode 126 into the concave areas betweenlobes 123, without damaging separator 124 or lobes 123, and withoutforming gaps between the separator 124 and electrodes 122 and 126, evenduring high speed assembly processes. Manufacture is facilitated evenmore when no radius R_(1a) is smaller than 0.060 inch (0.76 mm).Similarly, when second electrode 126 is a solid body, second electrode126 comprises a plurality of lobes 127, which may have the samedimensions or have different shapes and/or sizes, and a plurality of theconcave areas is formed between adjacent lobes 127 of second electrode126.

Each lobe 123 has a radial center line, extending from longitudinal axis36 of cell 110. The distance w₁, perpendicular to the radial centerline, from the radial center line to the internal surface of firstelectrode 122 increases as the radial distance from longitudinal axis 36increases. This, too, facilitates the proper insertion of separator 124and second electrode 126 into the concave areas between adjacent firstelectrode lobes 123.

Another cell with increased electrode surface area is shown in FIG. 4,which is similar to the cross section in FIG. 3. Cell 210 in FIG. 4 issimilar to cell 110 in FIG. 3 except for the sizes and shapes of firstand second electrodes 222 and 226, their corresponding lobes 223 and227, and separator 224. In cell 210 the radius Rib of the convex ends oflobes 223 has been substantially increased compared to cell 110. Theradius Rib of cell 210 is at least 0.030 inch (0.76 mm). This furtherfacilitates assembly of separator 224, avoids gaps between separator 224and electrodes 222 and 226, and avoids damage to both separator 224 andlobes 223. Cell manufacture is further aided when radius Rib is at least0.060 inch (1.52 mm). As with the embodiment in FIG. 3, all firstelectrode lobes 223 can have the same dimensions, or lobes 223 may havedifferent shapes and/or sizes. While cell 210 has a reduced electrodeinterfacial surface area compared to cell 110, such surface areareductions may be necessary in order to make high quality, reliablecells at acceptable speeds for large scale manufacturing.

In another variation of the embodiment shown in FIG. 4, second electrode226 is a solid body and first electrode 222 is not a solid body. Firstelectrode 222 is inserted into the housing after second electrode 226and separator 224. In yet another variation, both electrodes are solidbodies. Either electrode may be inserted into the can first, or both maybe inserted together, with separator 224 between them.

When second electrode 226 is a solid body, the external surface ofsecond electrode 226 and the internal surface of the upstanding can wall12 define a cavity into which first electrode 222 is disposed. Secondelectrode 226 comprises a plurality of radially outward extending lobes227. The end of each lobe has at least one radius R_(2b). Adjacent lobes227 form a plurality of concave areas in the external surface of secondelectrode 226. When first electrode 222 is not a solid body, theinternal shape of first electrode 222 is generally defined by the shapeof the external surface of second electrode 226 in combination withseparator 224. Each of the concave areas between adjacent secondelectrode lobes 227 has at least one radius R_(2a), and whether thefirst electrode 222 is a solid body or not, each radius R_(2a) is atleast 0.030 inch (0.76 mm). Each lobe 227 has a radial center line,extending from longitudinal axis 36 of cell 210. The distance w₂,perpendicular to the radial center line, from the radial center line tothe external surface of second electrode 226 decreases as the radialdistance from longitudinal axis 36 increases. Lobes 227 and the concaveareas formed between them may have the same dimensions, or they may havedifferent shapes and/or sizes.

FIGS. 5 through 9 show yet other embodiments of the present invention.All are cross sections similar to those in FIGS. 3 and 4. Cells 310,410, 510, 610, and 710 have: first electrodes 322, 422, 522, 622, and722, respectively; first electrode lobes 323, 423, 523, 623, and 723,respectively; separators 324, 424, 524, 624, and 724, respectively;second electrodes 326, 426, 526, and 726, respectively; and secondelectrode lobes 327, 427, 527, 627, and 727, respectively. Like cells110 and 210, either or both electrodes in cells 310, 410, 510, 610, and710 may be solid bodies.

While the electrochemical battery cells, as shown and described herein,are Cylindrical alkaline cells, it should also be appreciated that theteachings of the present invention can be applied to various types ofbattery cells of other electrochemical systems and having various sizesand configurations.

The following are useful general guidelines in designing cells accordingto the present invention, though there are interactions that must alsobe considered:

-   (1) The volume ratio of the negative and positive electrodes should    be based on the desired ratio of theoretical capacities of the    negative and positive electrode active materials. This is normally    established independently, based on safety, leakage and discharge    performance criteria.-   (2) Maximize electrode interfacial surface area for the best high    power/high rate discharge performance.-   (3) Improvements in high power/high rate discharge performance as a    result of increased electrode interfacial surface area will be    offset by the increase in the volume of separator required and a    corresponding decrease in volumes of active materials.-   (4) Improvements in high power/high rate discharge performance may    be offset by a reduction in the volumes of active materials if solid    electrodes must be strengthened (e.g., by adding binder or otherwise    changing electrode formulations).-   (5) If electrode interfacial surface area is increased by more than    necessary to achieve the desired high power/high rate performance    levels, capacity will be lower, especially at lower power/lower rate    discharge.

According to guideline (3) above, the possible improvement in high ratedischarge capacity from increasing the electrode interfacial surfacearea is partially offset by reduced amounts of active materials, becauseof the increased amount (and volume) of the separator needed to coverthe interfacial areas of the electrodes. The effects of increasedseparator volume are much more apparent at low discharge rates, wheredischarge efficiency is much better, and the amount of active materialsin the cell is a more important factor in determining dischargecapacity. As the interfacial surface area increases, the dischargecapacity at relatively low rates decreases because the capacity loss dueto reduced active material volume is greater than the small amount ofcapacity gained due to increased interfacial surface area and improveddischarge efficiency. Since many consumer alkaline cells are used in awide variety of devices that discharge cells across a wide range ofrates (e.g., from about 20 mA to 1000 mA and beyond), it is oftendesirable that there be a balance between increased surface area andincrease separator volumes. The importance of minimizing separatorvolume increase is also greater in battery types that do notspecifically target devices requiring high power discharge.

The present invention can use a variety of separator types andinsertion, assembly, and application processes. The following aregeneral considerations in selecting materials, forms, and processes. Thematerial type must be one that is suitable for performing the intendedseparator functions in a cell of the electrochemical system in which itis to be used. The amount of separator material should be minimized tomake the maximum amount of volume available for active materials. Theamount of additional separator required for a given increase inelectrode interfacial area can be minimized in a number of ways. Foldsin the separator and overlaps in the separator and between separator andother insulators within the cell should be minimized, and the separatorshould be as thin as possible. There are limits imposed by the naturesof the separator materials and the processes for making and assemblingthe separator into the cell. For example, the increased complexity ofthe shape of a first electrode with increased interfacial surface mayrequire more folds in the separator when the separator is formed from aflat sheet. If the separator is too thin, short circuits through theseparator can occur, and manufacturers often push the limits ofseparator thinness in conventional cells with a smooth electrodeinterface surface. The electrode edges and smaller radii to which theseparator must conform in a cell with increased interfacial area tend toincrease the minimum separator thickness required. Clearances forseparator and separator insertion tooling may be smaller when the cavityin first electrode does not have a smooth, round shape, thus placingadditional constraints on the selection of suitable materials andprocesses. While the present invention does not necessarily require aparticular separator material, form, or process for assembly,application, or insertion into the cell (referred to below as separatorassembly), the above considerations and other advantages anddisadvantages must be taken into account. For example, if the separatormaterial is in the form of a sheet, there are advantages anddisadvantages to preforming the separator to more closely match theshape of the cavity in the first electrode and reduce the volume ofseparator folds before the separator is inserted into the cell. Forming(e.g., by thermoforming) a separator of fairly uniform thickness toclosely match the shape of the cavity in the positive electrode isanother alternative. Spraying a coating onto the internal surface of thefirst electrode, before or after the first electrode is put into the canis yet another.

Regardless of the separator material, form, and assembly process, theshape and dimensions of the cavity in the first electrode to which theseparator must be placed and to which it must conform must beconsidered. Often those design features that facilitate separatorassembly tend to reduce electrode interfacial surface area. For example,to maximize electrode interfacial surface area, the shape of theinterfacial surface tends to have more projections or lobes, withsharper corners on the convex or protruding portions, smaller radii inthe concave or intruding portions, and smaller open areas betweenprotrusions. In contrast, optimum conditions for minimizing separatorvolume and facilitating separator assembly tend to include smoothsurfaces, without sharp corners, having the largest radii possible inall curved portions of the interfacial surface, and large open areasproviding the maximum clearance for assembling, inserting, or applyingthe separator. As with designing for maximum interfacial surface areaand minimum separator volume, there must be a balance between maximizinginterfacial surface area and the choices of separator material andassembly process.

Another consideration in design of a cell with increased electrodeinterfacial surface area is getting intimate contact between theseparator and each of the electrodes, without air pockets, or voids, atthe interfacial surfaces. Voids can result in incomplete utilization ofactive materials in the electrodes in the vicinity of the voids,particularly during discharge at higher rates. Voids between theseparator and the electrodes may also result in a reduced quantity of atleast one of the electrodes and/or less than an optimal match inelectrode heights in the cell. Voids can also result in tearing of theseparator if one electrode applies force against the separator at thevoid. The same factors that facilitate proper assembly of the separatoralso contribute to avoiding voids.

Manufacturability and durability of the solid electrodes are alsoconsiderations in the design of a cell with increased electrodeinterfacial surface area. Complex shapes are more difficult andexpensive to form and more difficult to control in manufacturing. Sharpprojections are more fragile, and material is more likely to break offduring the manufacturing process as well as during handling and use ofthe cell. Thin areas in the electrode also make it more fragile andsusceptible to breakage. In general, the same characteristics ofelectrode interfacial surface shape that contribute to proper separatorassembly are also advantageous for electrode manufacture and assembly.

Taking the above relationships into consideration, electrochemicalbattery cells of the present invention have an increased electrodeinterfacial surface area and improved high rate discharge performancethat is practical to manufacture and reliable under typical conditionsof shipping, handling, use and abuse. In one aspect of the invention noradius in any concave area of the interfacial surface of a solidelectrode is less than 0.030 inch (0.76 mm). In another aspect each lobehas a convex surface with no radius less than 0.030 inch (0.76 mm). Inanother aspect lobes do not increase in width from base to tip; thelobes may continually decrease in width from base to tip. Each of thesefeatures contributes to electrode manufacturability, electrodedurability, and separator assembly but places a limit on the maximumelectrode interfacial surface area that can be achieved.

In general, those compositions and materials, including those forelectrodes, electrolyte and current collectors, found to be suitable andpreferable for conventional cells to give good high rate and high powerdischarge performance will tend to be suitable and preferable in cellsmade according to the present invention.

As discussed above, electrodes with projections or lobes to increase theinterfacial surface area are typically more fragile than those in cell10 in FIGS. 1 and 2. Some cells have solid electrodes with a high degreeof strength and structural integrity because of the nature of thematerials used. In others the solid electrodes have lower strength andstructural integrity. When the solid electrodes contain mixtures ofdiscrete particles, they are more fragile than solid sheets of activemetal and materials that are sintered together into a rigid mass, forexample. A number of factors can contribute to the strength of suchelectrodes, each of which may be modified to improve the electrodestrength. This is illustrated in the following example of a cathode fora Zn/MnO₂ cell with an aqueous alkaline electrolyte; e.g., an LR6/AAtype cell. The principles disclosed can also be applied to other celltypes, both cells of the invention and other cells, with solidelectrodes comprising mixtures of particulate materials.

A common alkaline Zn/MnO₂ cell cathode comprises a mixture of MnO₂active material and particles of graphite, which is used to increase theelectrical conductivity of the electrode. The MnO₂ is often anelectrolytic manganese dioxide (EMD). Suitable alkaline cell grade EMDcan be obtained from Kerr-McGee Chemical Corp. (Oklahoma City, Okla.,USA) and Erachem Comilog, Inc. (Baltimore, Md., USA). Preferably the EMDis a high-potential EMD (pH-voltage of at least 0.86 volt) with apotassium content less than 200 ppm, as disclosed in InternationalPatent Publication No. WO 01/11703 A1, published 15 Feb., 2001. Thegraphite may be an alkaline grade graphite powder, an expanded graphite,or a mixture thereof. A suitable expanded graphite, according toInternational Patent Publication No. WO 99/00270, published 6 Jan. 1999,is available from Superior Graphite Co. (Chicago, Ill., USA). Themixture typically also comprises water (with or without electrolytesalt), and may also include small (typically less than 2 percent byweight) amounts of other materials, generally to improve performance insome way. Examples of such performance-enhancing materials includeniobium-doped TiO₂, as disclosed in International Patent Application No.WO 00/79622 A1, and barium sulfate.

Alkaline cell cathode mixtures that are suitable for use will havesufficient strength to hold together, without loosing significantamounts of electrode material from the surfaces of the formed cathodeduring manufacture, shipping, storage and use. Alkaline cell cathodescan be strengthened in a number of ways, either alone or in combination.Increasing the minimum cathode thickness will make the cathode stronger.

In some cells a binder is added to the cathode mixture to strengthen thecathode. The binder may also have some additional desirable properties.For example, the binder may function as a lubricant when the cathode isformed or may retain electrolyte in the cell, facilitating ion mobilityduring discharge. In general, a minimal amount of binder (or none) isused in order to maximize the amounts of active and electricallyconductive materials. When a binder is used it generally comprises about0.1 to 6, more typically 0.2 to 2, weight percent of the solidcomponents of the positive electrode mixture. Suitable binders foralkaline Zn/MnO₂ cathodes include monomers and polymers of materialssuch as acrylic acid, acrylic acid salts, tetrafluoroethylene, calciumstearate, acrylic acid/sodium sulfonate copolymer, and copolymers ofstyrene and one or more of butadiene, isoprene, ethylene butylene, andethylene propylene. Binder materials may be used alone or incombination. CARBOPOL® 940 (an acrylic acid in the 100% acid form fromB. F. Goodrich), Coathylene HA 1681 (a polyethylene from HoechstCelanese), KRATON® G1702 (a diblock copolymer of styrene, ethylene, andpropylene from Kraton Polymers Business), poly (acrylic acid-co-sodium4-styrene sulfonate) have been found to provide good electrode strength.Mixed binders, such as a mixture of CARBOPOL® 940 and either TEFLON®T30B or TEFLON® 6C (tetrafluoroethylenes from E. I. du Pont de Nemours &Co.), can be advantageous. When a mixture of these two materials isused, a CARBOPOL® to TEFLON® weight ratio of from 1:4 to 4:1 isadvantageous. In general, within this range, the higher the ratio, thestronger the cathode. For example, the cathode is stronger with aCARBOPOL® to TEFLON® weight ratio of 3:1 than with a ratio of 1:1 or1:3. When a CARBOPOL®/TEFLON® mixture is used, the binder level in thecathode may be about 0.2 to 2, preferably 0.2 to 1, weight percent,based on the solid, undissolved components in the cathode mixture.

The cathode may also be strengthened by applying a coating to thesurface of the cathode. Materials that are suitable for use as bindersmay be used for this purpose. The coating may penetrate to some extentinto the cathode to further bind the cathode material beneath thesurface. The coating material may also tend to absorb electrolyte,helping to keep the anode/cathode interface wet during discharge. Thecoating material may also function to some extent as a separatormaterial, providing improved mechanical contact between the separatorand the cathode. Poly (acrylic acid-co-sodium 4-styrene sulfonate) hasall of these advantages.

The amount of water in the mixture, generally from about 1.5 to 8.0percent, based on the weight of the solid, undissolved ingredients inalkaline cell cathodes prior to molding, affects electrode strength. Atypical range for use in making impact molded cathodes is 6 to 8percent. A typical range for use in ring molding is 1.5 to 6 percent,with 2 to 4 percent giving improved strength while better assuring goodcathode molding.

The percent solids packing in the cathode mixture is also a factor inthe cathode strength. The percent solids packing is determined bydividing the sum of (weight/real density) of solid components by theactual volume of the formed cathode. For typical alkaline cellcylindrical cathodes the packing can range from about 60 percent toabout 80 percent. High packing levels provide more active materials butlower efficiency on high rate discharge due to lower water levels andpoorer ion mobility in the cell. Though a relatively low packing levelis desirable to maximize high rate discharge capacity, high packing isdesirable to maximize cathode strength. The solids packing is typicallyabout 70 to 79 percent, with 72 percent being most typical in impactmolded cathodes and 75-79 percent being most typical in ring moldedcathodes. Cathode strength generally increases with increasing solidspacking, but processing considerations may introduce additionalconstraints for cells made using high speed processes. A number offactors can affect the solids packing. Included are: characteristics ofthe component materials, such as real density, intraparticle porosity,specific surface area, and particle size and shape distribution; theamount of water in the mixture during cathode forming; the force appliedduring cathode forming; the forming process used; and the amounts ofeach of the solid components. Electrolytic manganese dioxide (EMD) maybe used as the MnO₂, and expanded natural graphite may be used as thegraphite. The force applied during forming of the cathode will vary withthe method of forming (e.g., ring molding or impact molding), thecomposition of the cathode mixture, the size and shape of the cathode,and the desired solids packing. In general, the greater the moldingforce, the greater the cathode strength, up to a maximum achievablesolids packing level.

Alkaline cell cathodes are generally formed symmetrically around thelongitudinal axis of the cell; however, they may be non-symmetrical,either intentionally or as a result of variability in the manufacturingprocess. Accordingly, where electrode shapes are non-symmetrical alongthe longitudinal axis, the invention may be applied to individual lobes,concave areas, and convex areas. The invention advantageously applies toeach lobe, concave area, and convex base. Similarly, where electrodeshapes vary along the longitudinal axis, the invention may be applied toindividual cross sections normal to the longitudinal axis; the inventionadvantageously applies to each such cross section in the cell.

Two common methods of forming alkaline cell cathodes are ring moldingand impact molding. In ring molding one or more (usually 3 to 5) ringsare formed and then inserted into the can in a stack (one ring on top ofanother). Good physical and electrical contact between the can and thecathode are desirable. To achieve this the outside diameter of the ringsmay be made slightly larger than the inside diameter of the can toproduce an interference fit, or the rings may be slightly smaller thanthe can to facilitate insertion, after which the rings are reformedslightly by applying force to the inside and/or top surface, therebyforcing cathode mixture firmly against the can. In impact molding thedesired quantity of cathode mixture is put into the bottom of the canand molded to the desired dimensions using a ram that is inserted intothe center of the can. Both methods have advantages and disadvantages.In some cells a ring molded cathode gives better high rate dischargecapacity than an impact molded cathode. However, the cathode rings mustbe handled between molding and insertion into the can, generallyrequiring a stronger molded cathode than needed for impact molding. Inmaking cells with higher interfacial electrode surface area, the ringmolding process can have additional disadvantages. Because the formedelectrodes are typically more fragile than those in conventional cellssuch as cell 10 in FIGS. 1 and 2, other means of strengthening theelectrode may be necessary, as discussed above. If there are multiplestacked electrode rings in the cell, it may be necessary to orient allof the rings so the surfaces that coincide with the other electrode(e.g., the anode), adding complexity to the cell manufacturing process.

Impact molded cathodes are formed within the can and do not have to behandled separately, so the strength needed is generally much less thanfor ring molded cathodes. This can give the battery designer morefreedom in selecting a shape that will maximize the electrodeinterfacial surface area. It may also minimize or eliminate the need tostrengthen the cathode by means, such as adding binders, that canadversely affect cell discharge capacity.

The anode of an alkaline Zn/MnO₂ cell often comprises a mixture ofgelled zinc particles. The zinc may be in powder or flake form, or acombination of the two. An unamalgamated zinc alloy comprising bismuth,indium, and aluminum may be advantageous. Zinc powder, preferably havinga d₅₀ of about 110 μm, may be obtained from Umicore (Brussels, Belgium),and zinc flake (e.g., grade 5454.3) may be obtained from Transmet Corp.(Columbus, Ohio, USA). The anode also comprises water, potassiumhydroxide electrolyte, and a gelling agent. Acrylic acid in the 100%acid form, such as CARBOPOL® 940 from B. F. Goodrich Specialty Chemicals(Cleveland, Ohio, USA) is a common gelling agent. Small amounts of othermaterials may also be added to the anode mixture and/or electrolyte tominimize gas generation in the cell and/or enhance dischargeperformance. Examples of such materials include In(OH)₃, ZnO, and sodiumsilicate.

The total KOH concentration in the electrolyte in the completed cellwill generally be from about 36 to about 40 weight percent. The lowerpart of this range may be desirable for good high rate/high powerdischarge performance.

The anode of an alkaline Zn/MnO₂ cell of the present invention can beinserted into the cell in any suitable manner. The anode may be flowablewhen it is put into the cell and will flow by means of gravity to fillthe cavity in the cathode and separator. The anode could also bedispensed into the cell under pressure, e.g., by extrusion. This maytend to fill the anode cavity more completely, though there may be anincreased risk of damage to the separator, especially if there are voidsbetween the separator and the interfacial surface of the cathode.

In another embodiment of the invention, the second electrode may be thesolid electrode rather than the first electrode, as in the embodimentdescribed above. In such an embodiment the method of assembling the cellmay have to be modified. For example, if the solid second electrode isinserted first, it may have to be held in position while the firstelectrode is dispensed into the cavity between the second electrode andthe can. Alternatively, the first electrode may be dispensed into thecan, followed by insertion of the second electrode, with the separatordisposed thereon, forcing the flowable first electrode upward to fillthe cavity between the second electrode and the can.

In yet another embodiment of the invention, in which both the first andsecond electrodes are solid bodies, one of the electrodes may beflowable until after the electrodes and separator are assembled into thecell, when the flowable electrode is rendered solid. Alternatively, thefirst and second electrodes may be assembled together with the separatorbefore insertion into the can. In such an embodiment the interfacialsurface of one electrode would be a close match to the shape of theinterfacial surface of the other electrode to provide intimate contact,without voids, between the separator and both electrodes. To assemblethe electrodes and separator outside the can, the first electrode may becomprised of two or more sections that are mated together around theseparator and the second electrode. In an alternative method, theinterfacial surfaces of both electrodes may be vertically tapered tofacilitate insertion of one electrode into the cavity in the other.

A cell of the present invention may also comprise one or more additionalelectrodes, with the electrodes arranged in a coaxial manner, as long asat least one interfacial surface has a plurality of lobes, according tothe invention, for increasing the interfacial surface area. Theadditional electrode may be disposed outside the first electrode orinside the second electrode, or two additional electrodes may be used,one outside the first electrode and one inside the second electrode. Theelectrodes may have alternating polarities. This type of arrangementfurther increases the total interfacial surface area between anodes andcathodes.

The current collectors used in the invention may be any currentcollectors that would be recognized as suitable. Current collectors willbe stable in the internal environment of the cell, will have a suitableelectrical conductivity, and will have good electrical contact with theelectrode. When the outer electrode is a solid body, the can oftenserves as its current collector. Current collectors may also be otherstructures of varying shapes and numbers, depending in part on theelectrode material, shape, and location in the cell. For example, one ormore pins, nails, strips, or screens, or a combination thereof, may beused.

Table 1 below summarizes electrode dimensions for exemplary LR6/AA sizecells with electrodes according to FIGS. 3-9, as well as the ratios ofd₁:d₂, surface area, and volume. TABLE 1 Drawing Cell 110 210 310 410510 610 710 r₁ (mm) 6.15 6.21 5.68 5.93 5.93 5.93 5.93 r₂ (mm) 2.16 3.313.53 3.57 3.46 3.31 3.21 d₁ (mm) 3.30 4.01 3.14 3.54 3.44 3.80 3.72 d₂(mm) 0.56 0.50 1.03 0.78 0.78 0.78 0.78 w₁ with increasing incr. incr.incr. incr. incr. incr. incr. distance from center R_(1a) (mm) 1.52 1.642.02 1.89 1.89 1.64 1.64 R_(1b) (mm) near 0 2.40 1.64 2.15 1.89 2.402.15 d₁:d₂ 5.90 8.02 3.05 4.56 4.44 4.90 4.80 1^(st) electrode 1867 16981517 1586 1603 1603 1622 surface area (mm²) 1^(st) electrode 3384 29913018 2891 2930 3143 3188 volume (mm³)

While the cells in Table 1 are LR6/AA size cells, the invention issuitable for use in other cell sizes as well, including LR03/AAA,LR14/C, and LR20/D sizes. The typical can inside diameters (and cathodeoutside diameters) for these cell sizes range from about 10 to about 35mm, compared to about 12.7 to 14.0 for LR6/AA cells. In general, thereis more freedom in designing cells in which the cathodes are larger indiameter. For example, it is possible to increase the electrodeinterfacial surface area by increasing the number of lobes in theelectrode surface. There is a limit to the number of lobes that ispossible without having surfaces that are not conducive to high speedmanufacturing. The number of lobes that can be used in cells accordingto the invention increases as the outside diameter of the cathodeincreases. For example, in an LR03/AAA cell it is difficult tomanufacture a cell with more than 3 lobes, while in an LR20/D cell 6lobes or more are possible.

In cells made according to the invention, the minimum radial thickness(d₂) of the cathode will generally be less than the radial thickness ofthe cathode of a cell such as cell 10 in FIG. 2. If d₂ is too large,increasing the electrode interfacial surface area enough to realize asignificant improvement in high rate discharge efficiency will reducethe volume available for the anode to the point that the theoreticalinput capacity will be too low and/or the ratio of anode to cathode willbe outside the range desired for safety. If d₂ is too small, the cathodewill be too fragile. Once d₂ is established, the radial distances fromthe center of the cell to the ends and bases of the lobes (r₂ and r₁,respectively) must be selected so as to provide a sufficient increase ininterfacial surface area without requiring too much additional separatoror creating shapes that make assembly of the separator and properdispensing of the anode into the cell too difficult. In LR6/AA sizecells d₂ is advantageously at least 0.40 mm but no greater than 1.20 mm,r₂ is advantageously at least 3.20 mm but no greater than 3.70 mm, andr₁ is advantageously at least 5.60 mm but not more than 6.30 mm.

LR6/AA cells of the invention will typically have a ratio of theelectrode interfacial surface area to the cathode volume of about 0.45mm²:1 mm³ to 0.60 mm²:1 mm³, advantageously about 0.49 mm²:1 mm³ to 0.60mm²:1 mm³. If the ratio is too high, cell manufacture will be difficult.If it is too low, the increase in interfacial surface area over a cellhaving no lobes on the interfacial surface will be small.

In order to maximize discharge efficiency, it would be desirable to havea very uniform shallow cathode depth from the interfacial surface withthe anode throughout the cathode. To manufacture a cell with a cathodehaving such a shape as well as a high interfacial surface area comparedto that in a cell similar to the one in FIG. 2 would be very difficult,as disclosed above. Therefore, manufacturing considerations will placepractical limits on the uniformity of cathode depth from the interfacialsurface of the cathode. A uniform lobe width from base to tip and ad₁:d₂ ratio of about 2:1 would be ideal for maximizing dischargeefficiency. As described above and illustrated in FIGS. 3-9, cells thatare practical to manufacture do not have uniform lobe widths. Such cellswill often have a d₁:d₂ ratio of at least 2.5:1. Cells with a d₁:d₂ratio of at least 3.0:1 are more easily made, and cells with a ratio ofat least 4.0:1 are even more easily manufactured. Because impact moldedcathodes do not have to be handled outside the cell, d₂ can be madesmaller than in impact molded cells, and cells with d₁:d₂≧6.5, or even≧7.0, are practical. If d₁:d₂ is greater than 8.1:1, the high ratedischarge improvement resulting from increasing the interfacial surfacearea may be offset more than desired because of nonuniform discharge ofthe cathode.

The features and advantages of the invention are illustrated in view ofthe following examples.

EXAMPLE 1

Conventional LR6/AA alkaline Zn/MnO₂ cells were made with a design asshown in FIGS. 1 and 2 and described above.

Cathode mixture was made by blending together electrolytic manganesedioxide (EMD) and expanded graphite, in a weight ratio of 17:1, smallamounts (less than 1 weight percent each) of BaSO₄ and Nb-doped TiO₂,7.8 weight percent of 45 weight percent KOH solution, and 1.6 weightpercent of deionized water. Cathodes were impact molded into steel canswith an inside diameter of 0.528 inch (13.4 mm). The cans were 0.010inch (0.254 mm) thick, with nickel plating on the outside surface, andcoated with a graphite coating on the inside surface. Nominally 10.8 gof cathode mixture was put into each cell, and the cathodes were moldedto a height of 1.674 inches (42.52 mm) and an inside diameter of 0.370inch (9.40 mm), with 73.2 volume percent solids packing. The radialthickness of the molded cathodes was 0.079 inch (2.01 mm), the area ofthe inner surface of the cathode was 1.946 in² (1255 mm²), and thecathode volume was 0.1867 in³ (3060 mm³).

After forming the cathode in the can, the separator was cut, formed intoa roughly cylindrical shape, and inserted into the cavity formed by theinner surface of the cathode and the bottom of the can. The separatorwas made from 0.004 inch (0.10 mm) thick grade VLZ 105 from NipponKodoshi Corporation of Kochi-ken, Japan, and was 2.244 inches (57.00 mm)long×2.165 inches (54.99 mm) high. The cut separator was scrolled alongits length around a mandrel. The scrolled separator was folded inward atthe bottom to form a basket shape to cover and conform to the sides andbottom of the inner surface of the cathode and can bottom. The formedseparator was heated to seal the separator layers and maintain its shapeduring insertion into the cell.

After inserting the formed separator into the cell, 1.19 g of 37 weightpercent KOH in deionized water was added to each cell to soak theseparator.

Anode gel mixture was made by blending together the following (allpercentages based on weight): 69.00 percent zinc alloy powder, 0.44percent gelling agent, 29.39 percent electrolyte solution, 0.02 weightpercent In(OH)₃, and 1.15 weight percent 0.1 N KOH. The electrolytesolution contained 40 percent aqueous KOH (96.7 percent), ZnO (3.0percent), and sodium silicate (0.3 percent). 6.04 g of anode mixture wasdispensed into the cavity in the separator in each cell.

The nominal ratio of anode to cathode theoretical input capacities ineach cell was 0.99:1, based on an assumed 1.33 electron discharge of theEMD. The nominal overall KOH concentration in each cell (anode, cathode,and separator) was 37.3%.

The cells were closed by placing an anode collector assembly, includinga current collector nail, a seal, and a cover, into the open end of thecan, followed by a negative terminal cover. The collector assembly andterminal cover were held in place and the cell sealed by crimping thetop edge of the can inward and over the top of the seal and terminalcover.

The cells were completed by welding a positive terminal cover to thebottom of the can and placing a label over the outside of the can,extending over ends of the cell.

EXAMPLE 2

Cells were made in the manner described in Example 1, except for thefollowing. The cross-sectional shape of the cathode corresponded to thatof cell 210 in FIG. 4. Table 1 summarizes key dimensions and dimensionalrelationships (cell 210). Because of the increased electrode interfacialsurface area compared to Example 1, more separator was needed. The cutseparator was 3.07 inches (77.97 mm) long×2.165 inches (54.99 mm) high,and the amount of electrolyte added after separator insertion wasincreased to 1.29 g.

EXAMPLE 3

Cells were made in the manner described in Example 2, except for thefollowing. The cross-sectional shape of the cathode corresponded to thatof cell 310 in FIG. 5, and the electrode dimensions were those shown forcell 310 in Table 1. The cut separators were 3.07 inches (77.97 mm)long×2.165 inches (54.99 mm) high, and the amount of electrolyte addedafter separator insertion was 1.29 g.

EXAMPLE 4

Cells were made in the manner described in Example 2, except for thefollowing. The cross-sectional shape of the cathode corresponded to thatof cell 710 in FIG. 9, and the electrode dimensions were those shown forcell 710 in Table 1. The cut separators were 3.07 inches (77.97 mm)long×2.165 inches (54.99 mm) high, and the amount of electrolyte addedafter separator insertion was 1.29 g.

EXAMPLE 5

Cells from Examples 1, 3, and 4 were discharged continuously at 1000milliwatts to 1.0 V at 21° C. Other cells from Examples 1, 2, and 3 weredischarged continuously at 1000 milliamps to 1.0 V at 21° C. The resultsare summarized in Table 2 below. The cathode surface areas and dischargedurations are normalized (indexed), with the surface areas and durationsfor the comparative cells of Example 1 set at 100%. TABLE 2 Example 1 23 4 Drawing FIG. 2 4 5 9 Cell 10 210 310 710 Cathode surface area 11881698 1517 1622 (mm²) Cathode surface area 100  143 128  137 (%) 1000 mWdischarge 100 — 106  125 duration (%) 1000 mA discharge 100  124 109 —duration (%)

As shown in Table 2, the cathode interfacial surface area was increasedover that of the conventional cells in Example 1 by 43% in Example 2,28% in Example 3, and 37% in Example 4. As discussed above, theadvantage of increased surface area is partially offset by an increasein separator volume and a corresponding reduction in the amount ofactive materials that can be put into a cell.

EXAMPLE 6

Cells were made in the manner described in Example 2, except for thefollowing. The cross-sectional shape of the cathode corresponded to thatof cell 110 in FIG. 3, and the electrode dimensions were those shown forcell 110 in Table 1. Processing problems were observed. Because of thesharp corners at the ends of the cathode lobes 123, it was difficult toget the separator 124 to conform to the inner surface of the cathode atthose corners and remain there. As a result, there were spaces, or gaps,between the cathode 123 and separator 124 in those areas, it wasdifficult to get all of the anode material into the anode cavity, andthe effective anode/cathode interfacial surface area was reduced. Theseproblems caused a high incidence of substandard cells, and the dischargecapacity of these cells was not tested. For best results, cells withsharp-ended electrode lobes have separator materials that will conformeasily to the lobe surfaces. For separator materials that are resilient(i.e., tending to spring back into a previous shape), such as those usedin Examples 1-4, rounded electrode lobe ends, particularly lobes whichhave no convex surface radius less than 0.030 inch (0.76 mm), are usefulto avoid the processing problems observed in Example 6. Processingproblems are further reduced when no convex surface radius is less than0.060 inch (1.52 mm).

EXAMPLE 7

LR20/D size cells were made in a manner similar to that for the cells inExample 3. The cathode shape was similar to that of cell 210 in FIG. 4and had the following nominal dimensions: r₁=0.549 inch (13.94 mm),r_(2=0.316) inch (8.03 mm), d_(2=0.090) inch (2.29 mm), R_(1a)=0.1505inch (3.82 mm), R_(1b)=0.248 inch (6.30 mm), and cathode height=2.035inch (51.69 mm). This resulted in a cathode volume of 1.421 in³ (23,298mm³) and an interfacial surface area of 7.020 in² (4,529 mm²), or 127%of the interfacial surface area of a conventional LR20/D size cell witha cylindrical cathode of 0.867 inch (22.02 mm) inside diameter. Eventhough the electrode dimensions of the cells in Example 7 were notoptimized, and the separators were not properly formed, leaving a gap ofabout 0.05 inch (1.27 mm) between the separator and the cathode at thebases of the cathode lobes, the discharge durations of these cells at1000 mA to 1.0 V averaged about 116% of conventional cells.

As demonstrated in the above examples, cells made according to theinvention provide improved high rate discharge duration overconventional cells and avoid shortcomings of previous methods to do so.

It will be understood by those who practice the invention and thoseskilled in the art that various modifications and improvements may bemade to the invention without departing from the spirit of the disclosedconcept. The scope of protection afforded is to be determined by theclaims and by the breadth of interpretation allowed by law.

1. An electrochemical battery cell comprising: a housing with anupstanding side wall; a first electrode comprising a first activematerial; a second electrode comprising a second active materialdisposed within the first electrode; a separator disposed between thefirst and second electrodes; an electrolyte; and a cell longitudinalaxis; wherein: at least one of the first and second electrodes comprisesa solid body having a surface with a plurality of radially extendinglobes, each having a base, a tip and a radial center line, that form aplurality of concave and convex areas in its surface; each concave andconvex area in the surface of the solid body has no radius less than0.76 mm; and each lobe has a width that continually decreases as adistance along its radial center line from its base to its tipincreases.
 2. The cell defined by claim 1, wherein each concave area hasno radius less than 1.52 mm.
 3. The cell defined by claim 2, whereineach convex area has no radius less than 1.52 mm.
 4. The cell defined byclaim 1, wherein the first electrode comprises a solid body having aplurality of radial extending lobes.
 5. The cell defined by claim 4,wherein: the first electrode has a minimum radial thickness d₂; eachlobe in the first electrode has a width d₁, measured between two pointson its surface, the two points being on opposite sides of the lobe at aradial distance from the longitudinal axis equal to an average of aradial distance from the longitudinal axis to an outermost point on thesurface of the first electrode and a radial distance from thelongitudinal axis to an innermost point on the surface of the firstelectrode; and a ratio d₁:d₂ is greater than 2.5:1 but not greater than8.1:1.
 6. The cell defined by claim 5, wherein the ratio d₁:d₂ is atleast 3.0:1.
 7. The cell defined by claim 6, wherein the ratio d₁:d₂ isat least 4.0:1.
 8. The cell defined by claim 7, wherein the ratio d₁:d₂is at least 6.5:1.
 9. The cell defined by claim 4, wherein the secondelectrode is non-solid.
 10. The cell defined by claim 1, wherein: thesecond electrode comprises a solid body with an external surface; thefirst electrode has a minimum radial thickness; the first electrode hasan internal surface with a plurality of lobes defined by an externalsurface of the second electrode solid body and the separator; each firstelectrode lobe has a width, measured between two points on the theinternal surface, the two points being on opposite sides of the lobe ata radial distance from the longitudinal axis equal to an average of aradial distance from the longitudinal axis to an outermost point on theinternal surface and a radial distance from the longitudinal axis to aninnermost point on the internal surface; and a ratio of each firstelectrode lobe width to its minimum radial thickness is greater than2.5:1 but not greater than 8.1:1.
 11. The cell defined by claim 1,wherein the cell is a cylindrical cell.
 12. The cell defined by claim11, wherein the housing has an inside diameter from 10 mm to 35 mm. 13.The cell defined by claim 1, wherein the cell is a noncylindrical celland the housing has an inside width through a longitudinal axis of thecell from 10 mm to 35 mm.
 14. An electrochemical battery cellcomprising: a housing with an upstanding side wall; a first electrodecomprising a first active material and having an internal surface; asecond electrode comprising a second active material disposed within thefirst electrode and having an external surface adjacent to the internalsurface of the first electrode; a separator disposed between the firstand second electrodes; an electrolyte; and a cell longitudinal axis;wherein: the first electrode comprises a solid body having a pluralityof radially extending lobes, each having a base, a tip and a radialcenter line, that form a plurality of concave and convex areas in itsinternal surface; each concave area has no radius less than 1.52 mm; thefirst electrode has a minimum radial thickness d₂; each lobe has a widthd₁, as measured between two points on the internal surface, each of thetwo points a radial distance from the longitudinal axis equal to anaverage of a radial distance from the longitudinal axis to an outermostpoint on the internal surface and a radial distance from thelongitudinal axis to an innermost point on the internal surface; a ratiod₁:d₂ is greater than 2.5:1 but not greater than 8.1:1; and each lobewidth continually decreases as a distance along its radial center linefrom its base to its tip increases.
 15. The cell defined by claim 14,wherein each convex area in the surface of the first electrode has noradius less than 1.52 mm.
 16. The cell defined by claim 14, wherein thecell is a cylindrical cell, and the housing has an inside diameter from10 mm to 35 mm.
 17. The cell defined by claim 16, wherein the cell is anoncylindrical cell, and the housing has an inside width perpendicularto the longitudinal axis from 10 mm to 35 mm.
 18. An electrochemicalbattery cell comprising: a housing with an upstanding side wall; a firstelectrode comprising a first active material and having an internalsurface; a second electrode comprising a second active material disposedwithin the first electrode and having an external surface adjacent tothe internal surface of the first electrode; a separator disposedbetween the first and second electrodes; an electrolyte; and a celllongitudinal axis; wherein: at least one of the first and secondelectrodes comprises a solid body having a plurality of radiallyextending lobes, each having a base, a tip and a radial center line,that form a plurality of concave and convex areas in its surface; thehousing has an inside diameter no greater than 14.0 mm when the cell isa cylindrical cell and an inside width through a longitudinal axis ofthe cell no greater than 14.0 mm when the cell is a noncylindrical cell;and each concave and convex area has no radius less than 1.52 mm. 19.The cell defined by claim 18, wherein each lobe has a width thatcontinually decreases as a distance along its radial center line fromits base to its tip increases.
 20. The cell defined by claim 19,wherein: the first electrode has a minimum radial thickness d₂; eachlobe in the first electrode has a width d₁, measured between two pointson its surface, the two points being on opposite sides of the lobe at aradial distance from the longitudinal axis equal to an average of aradial distance from the longitudinal axis to an outermost point on thesurface of the first electrode and a radial distance from thelongitudinal axis to an innermost point on the surface of the firstelectrode; and a ratio d₁:d₂ is greater than 2.5:1 but not greater than8.1:1.